linux-mm wiki drop_caches doc is incomplete

[Todd Vierling <todd.vierling@oracle.com>]

I've traced back several bad uses of drop_caches to users and bloggers
who have used this doc page as a reference:

https://linux-mm.org/Drop_Caches

Of course, *we* know that this knob is intended for benchmarking and
debugging, but many less experienced system administrators don't. To a
more green admin, "free" memory might be considered a "good" thing,
never mind the entire purpose of reclaimable caches.

As this is not an editable moinmoin page, will someone with the proper
perms please fix this page, to include the next two paragraphs from the
kernel doc where it came from?:

===

https://www.kernel.org/doc/Documentation/sysctl/vm.txt

...

This file is not a means to control the growth of the various kernel
caches (inodes, dentries, pagecache, etc...)  These objects are
automatically reclaimed by the kernel when memory is needed elsewhere on
the system.

Use of this file can cause performance problems.  Since it discards
cached objects, it may cost a significant amount of I/O and CPU to
recreate the dropped objects, especially if they were under heavy use.
Because of this, use outside of a testing or debugging environment is
not recommended.

-- 
-- Todd Vierling <todd.vierling@oracle.com> +1-770-730-4426